1. Field of the Invention
The present invention relates to an image sensor and a driving circuit of a transfer transistor for charge transfer in a light receiving unit in the image sensor. More particularly, the present invention relates to a method of driving a transfer transistor which can constantly maintain a depletion degree of charges in photodiode reset at a low operating voltage condition and can very effectively reduce dark current and fixed pattern noise by additionally reinforcing a reset function, and an image sensor using the same.
This work was supported by the IT R&D program of Ministry of Information and Communication/Institute for Information Technology Advancement [2006-S-004-01, Silicon-based high-speed optical interconnection IC.]
2. Discussion of Related Art
Image sensors may be classified into a charge-coupled device (CCD) sensor and a CMOS image sensor, which basically utilize an electron-hole pair separated by light having a higher energy than a silicon bandgap. In image sensors, an amount of irradiated light is generally estimated by collecting electrons or holes.
A CMOS image sensor can be manufactured using a conventional CMOS semiconductor manufacturing process as is, and may include a photodiode and a transistor in each image pixel, which is similar to a general CMOS device. Also, it may integrate a pixel array and a circuit for processing and searching an image signal in the same chip. Thus, the CMOS image sensor may overcome a shortcoming of the CCD that has to have such an image signal processor in a separate chip, adapt various image sensor structures because of its integrated structure, and provide flexibility in performing various subsequent processes.
One of the structures widely used for a CMOS image sensor is a 4-transistor pixel structure, as illustrated in FIG. 1. In the above structure, a photodiode PD, which is a light receiving unit, and four NMOS transistors constitute one unit pixel. Among the four NMOS transistors, a transfer transistor TX serves to transfer a photo charge generated from the photodiode PD to a diffusion node region FD or deplete the photodiode PD in a reset step, a reset transistor Rx serves to emit charges stored in the diffusion node region FD or the photodiode PD for signal detection, a drive transistor Dx serves as a source follower transistor, and a switch transistor Sx serves to switch and address signals. The transfer transistor TX may be formed of a gate, a gate oxide layer and a p-type substrate, the photodiode PD may generally have an n− type or no type doping region and a surface p-type doping region, and a diffusion node 131 may be have an n+ type doping region.
In FIG. 1, the photodiode PD receiving light and a capacitor 118 disposed parallel thereto constitute a light receiving part, and the transfer transistor TX, which transfers the received electron, serves to transfer electrons generated by photons to the diffusion node 131.
The transfer transistor Tx serves as a transmission channel which moves electrons generated from the photodiode PD by applying a predetermined voltage to a gate 111 of the transfer transistor Tx to the diffusion node 131, or serves to reset the photodiode PD by completely eliminating the electrons. The diffusion node 131 includes a diffusion capacitance 114 and a gate capacitance of the drive transistor Dx, wherein the diffusion node 131 is reset by the reset transistor Rx. To be more specific, the diffusion node 131 is reset for correlated double sampling (CDS) right before bringing the electrons of the photodiode PD, or by applying a reset voltage to the diffusion node 131 for resetting the photodiode PD. In order to obtain a two-dimensional image, a voltage is applied to a gate 141 of the switch transistor Sx to select one column. In particular, one pixel is biased by a cu-rent source 150, which operates the drive transistor Dx and the switch transistor Sx so that the voltage of the diffusion node 131 is read out to an output node 142.
The illustrated CMOS image sensor having four transistors transfers photon-induced carriers accumulated in the photodiode to the floating diffusion node after the photodiode reset and detects the amount of the photon-induced carriers by voltage drop of the diffusion node. Here, in order to accurately and uniformly detect the amount of the accumulated photon-induced carriers, a transfer operation in the same level as the reset of the transfer transistor is needed. Thus, the conventional CMOS image sensor having four transistors employs a complete reset-type pinned photodiode structure for stable reset and transfer operations. The pinned photodiode is a diode using a state without a voltage variation by fully depleting mobile charges in the photodiode. In this case, ideally, a photodiode voltage is always pinned to a specific value irrespective of an external bias environment such as a voltage of the diffusion node, and thus reset and transfer conditions according to the transfer transistor operation may be always maintained on a specific level.
However, characteristics of the conventional 4-transistor CMOS image sensor may deteriorate due to a decrease in operating voltage or a change in process conditions. In particular, this is because the reset and transfer conditions may depend on conditions of the transfer gate voltage and voltage of the diffusion node. These will now be described in detail.
In a conventional driving method using a power supply voltage VDD as a turn-on voltage of a transistor, the voltage of the floating diffusion node in reset is VDD-Vth_RX that is obtained by subtracting a threshold voltage Vth_RX of the reset transistor RX from a gate voltage VDD of the reset transistor. Also, a difference between the gate voltage VDD of the transfer transistor and the gate voltage VDD-Vth_RX of the floating diffusion node becomes a threshold voltage Vth_RX of the reset transistor RX. Generally, substrate doping conditions of the reset and transfer transistors are the same as each other, and thus the threshold voltage of the transfer transistor is similar to that of the reset transistor (Vth_RX=Vth_TX).
The state according to the condition described above corresponds to the boundary between a linear region and a saturation region of a general metal-oxide layer-semiconductor (MOS) transistor, in which an edge of the transfer transistor adjacent to the floating diffusion node begins to be turned on by the definition of the threshold voltage Vth. Accordingly, at the time when the edge of the transfer transistor adjacent to the floating diffusion node is turned on, a specific amount of electrons may suddenly overflow into a channel region of the transfer transistor from the floating diffusion node, and thus a voltage difference in the floating node based on variations of the electron amount becomes relatively increased due to the capacitance of the diffusion node. Also, the electron amount coming over from the floating diffusion node causes a great change in the small difference of the threshold voltage due to the process characteristics of the transfer transistor and the reset transistor. Such non-uniformity of the electron amount coming over from the floating diffusion node causes irregular reset conditions, which may result in poor image quality. Moreover, when the electron comes over to the channel region of the transfer transistor from the floating diffusion node, the photodiode is not operated in the full depletion condition to reduce the reset voltage of the photodiode, and thus the electron remaining in the photodiode serves as a source generating a dark current.
The problems that may be caused by unstable reset and transfer operations of the transfer transistor may include poor noise characteristics, such as an increase in dark current and fixed pattern noise.
Further, in the general reset operation, the reset transistor RX is turned on, thereby maintaining the floating diffusion node as a low impedance node, and the voltage is almost the same as VDD-Vth_RX. On the contrary, in the transfer operation, when the reset transistor RX is turned-off, the floating diffusion node is at a high impedance level, and thus the voltage of the floating node becomes lower than VDD-Vth_RX by flowing the electrons in the channel region of the reset transistor RX into the floating node due to clock feedthrough. Then, the voltage becomes greater due to capacitive coupling depending on the gate voltage of the transistor. In this process, the voltages of the floating node in the reset and transfer operations are different from each other. Such different voltage levels can inhibit dark current and other noises by employing a pinned photodiode which is fully depleted (i.e., completely reset), that is, employing a structure which may fully deplete the pinned photodiode by driving a pixel in the condition for fully depleting the photodiode. Notwithstanding this, a structure that may fully deplete the pinned photodiode has not been disclosed.
However, recently, according to a decrease in scale and operating voltage of semiconductor processes and devices, the voltage of the floating diffusion node has gradually decreased. Thus, pinning voltage of the pinned photodiode has also decreased, and the photodiode is operated in a condition of non-fully depleted pinned photodiode (NFD-PPD) (Bongki Mheen et al., “Operation Principles of 0.18-μm Four-Transistor CMOS Image Pixels with a Non-fully Depleted Pinned Photodiode”, IEEE Trans, Electron Devices, col. 53, no. 1, 2006). In order to prevent such a phenomenon, the pinning voltage has to be lowered according to the decrease in operating voltage. However, the decrease is limited because it may make pixel characteristics such as well capacity worse.
Also, since a specific level of voltage barrier exists between the pinned photodiode and the channel of the transfer transistor, a difference between the pinning voltage and the voltage of the floating diffusion node has to be enough to sufficiently lower such a barrier in turning-on the transfer transistor. If the barrier is not sufficiently lowered, the pinned photodiode is not completely reset, which may result in more serious problems, as described above. In other words, the decrease in operating voltage represented as a power supply voltage VDD generally may reduce the difference between the pinning voltage and the voltage of the floating diffusion node, and may cause low well capacity and insufficient reset (i.e., depletion).
In conventional art, a method of compulsorily raising a voltage of a floating diffusion node from VDD-Vth, a common voltage, to VDD by a voltage applied to the gate of the reset transistor Rx, which is formed by applying a boosting circuit, and a method of sufficiently and rapidly raising the voltage of the floating diffusion node to VDD using a PMOS transistor, rather than a conventional NMOS transistor, as the reset transistor are disclosed.
However, the method using the voltage boosting circuit may deteriorate reliability of a gate oxide layer by applying a voltage higher than a general operation condition, and the method using the PMOS transistor as the reset transistor Rx may reduce a fill factor because the PMOS transistor occupies a larger area than the NMOS transistor and have a twice higher noise level than the NMOS transistor. Also, despite the improvement of the characteristic on the condition of complete reset, this method has limitations.